Estrutura e propriedades elásticas das fases alpha e gama do ZrW2O8

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

The crystalline structure and some elastic properties of the alpha and gamma phases of zirconium tungstate, ZrW2O8, were calculated according to Density Functional Theory using the B3LYP functional (DFT/B3LYP). The structure of alpha ZrW2O8, was optimized at different pressures and its elastic constants were estimates. The interatomic iterations, ranked in terms of bond compressibilities, decrease according to the sequence W-O >Zr W >Zr-O. The tetrahedra around tungsten atoms are found to be much stiffer than the ZrO6 octahedra. These latter are, in fact, more compressible than the alpha ZrW2O8 unit cell. The elastic constants calculated in the athermal limit are in excellent agreement with recent experimental results obtained near 0 K. The compression mechanism around W1 and W2 atoms is quite different. While the former can be described essentially in terms of a correlated polyhedral rotation, the latter involves correlated rotation of the first coordination polyhedra and translation of WO4 units downward along the <111>axis. As far as these modes of deformation should bear some resemblance to the low-energy modes responsible for the negative thermal expansion in zirconium tungstate, this result can shed some light on the microscopic mechanism behind this phenomenon. The structure of gamma ZrW2O8 was optimized at ambient pressure and for V/V0 = 0.97 (threshold of the gamma to amorphous phase transition) aiming to study the evolution of the structure of this phase with the pressure and the pressure induced amorphization mechanism. A reduction of 3 % in the volume of the unit cell leads to a variation greater than 1,5 % in the interatomic distances between the terminal atoms and the tungsten of the nearest tetrahedron. Therefore, it is expected that the reduction of 15 % in the molar volume with the gamma to amorphous transition promotes the formation of new W-O bonds, which would be responsible for the metaestable retention of the amorphous phase upon pressure release.

ASSUNTO(S)

tungstato de zircônio propriedades elásticas teoria do funcional da densidade zirconium tungstate materiais cerâmicos expansão térmica negativa amorfização induzida por pressão ceramic materials density functional theory pressure-induced amorphization elastic properties engenharia de materiais e metalurgica megative thermal expansion

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